In coherent optical transmission system, chromatic dispersion accumulates during fiber transmission and causes severe inter-symbol-interference (ISI), which brings severe degradation to system. During initialization of the digital coherent receiver, a robust and accurate CD estimation is vital to set the frequency-domain (FD) CD compensation filter. If the CD estimation fails with a large estimation error, none of the subsequent equalization and synchronization stages could work. This would result in a total failure of the transponder.
Currently, CD estimation is performed by non-data-aided, i.e. blind estimation techniques under implementation constraint conditions such as bandwidth limitation and sampling rate. The estimation is performed by scanning over the whole possible CD range and thus takes a lot of time and slows down the initialization speed of the digital coherent receiver as all other processes have to wait until it is finished.
The following two problems have been observed with the existing CD estimation:
Changing the step width towards a finer resolution, i.e. a smaller CD scanning step, does not improve the estimation precision. The estimation performance is limited and a more precise and more robust estimation could not be obtained.
Changing the step width towards a more coarse resolution, i.e. a larger CD scanning step, is desirable for a faster scanning procedure but runs into the risk of missing the area of optimum CD estimates.
The properties of the observations in A) and B) conclude that the precision of the estimation cannot be increased and that the complexity and the estimation time cannot be speeded up.